ACI Surface Preparation & Finishing Techniques 3 — Questions and Answers
Question 1: What type of finish is achieved by dragging a stiff-bristled broom across fresh concrete?
- Glass finish
- Broom finish (Correct answer)
- Exposed aggregate finish
- Salt finish
Correct answer: Broom finish
A broom finish creates a textured, slip-resistant surface by dragging bristles across the concrete before it fully sets.
Question 2: An exposed aggregate finish is achieved by:
- Polishing the hardened surface with diamond grinders
- Washing away the surface mortar to reveal embedded aggregate before it fully hardens (Correct answer)
- Stamping patterns into fresh concrete
- Applying a colored hardener to the surface
Correct answer: Washing away the surface mortar to reveal embedded aggregate before it fully hardens
Exposed aggregate finish involves removing surface mortar by washing or brushing while the concrete is still green, revealing the decorative coarse aggregate.
Question 3: Which chemical compound is commonly used as a surface retarder to achieve an exposed aggregate finish?
- Calcium chloride
- Sodium silicate
- Sugar-based surface retarder (Correct answer)
- Lithium bromide
Correct answer: Sugar-based surface retarder
Sugar-based surface retarders are sprayed on fresh concrete to delay hardening of the top mortar layer, allowing it to be washed away to expose aggregate.
Question 4: What is the recommended technique when applying a steel trowel finish for a hard, dense surface?
- Use heavy pressure on the first pass and light pressure on final passes
- Use light pressure on early passes and progressively increase pressure on later passes (Correct answer)
- Apply uniform moderate pressure throughout all passes
- Only trowel once in a single direction
Correct answer: Use light pressure on early passes and progressively increase pressure on later passes
Early troweling uses light pressure to smooth the surface; later passes apply increasing pressure to compact and densify the mortar layer.
Question 5: What does it mean when a finisher says the concrete has reached the 'thumbnail test' stage?
- The concrete is fully cured
- A fingernail pressed into the surface leaves only a slight indentation, indicating readiness for final finishing (Correct answer)
- The concrete has set too hard to finish
- Bleed water is still rising to the surface
Correct answer: A fingernail pressed into the surface leaves only a slight indentation, indicating readiness for final finishing
The thumbnail test checks firmness — a slight indentation without sinking indicates the surface is ready for final steel troweling.
Question 6: Salt finish on concrete is achieved by:
- Adding rock salt to the mix design
- Pressing rock salt into fresh concrete and washing it out after hardening (Correct answer)
- Applying salt water as a curing compound
- Using salt aggregate as coarse material
Correct answer: Pressing rock salt into fresh concrete and washing it out after hardening
Rock salt is embedded into the fresh surface and dissolved out after hardening, leaving a pitted, slip-resistant texture.
Question 7: Which of the following conditions is MOST likely to cause premature surface crusting that traps finishers?
- High humidity and low wind
- High temperature, low humidity, and high wind combined (Correct answer)
- Low temperature with no wind
- Overcast skies with moderate humidity
Correct answer: High temperature, low humidity, and high wind combined
Hot, dry, and windy conditions dramatically increase evaporation rate, causing the surface to crust before the concrete beneath has set.
ACI Surface Preparation & Finishing Techniques 3
What type of finish is achieved by dragging a stiff-bristled broom across fresh concrete?